I argued that although most of the goals have a 2015 deadline and are meant to have measurable targets (such as halting and reversing malaria), many of these goals are either immeasurable, or are not being measured at all.

How, therefore, can we verify whether the goals are being met, except by guesswork?

They concede that the same criticisms "have been made by many others before … including many professionals working for the UN system". So it does not seem debatable that what I am arguing is truthful: that progress (if any) towards the MDGs is not being measured as the UN claims.

I therefore find it hard to understand why Sachs and colleagues have sought to refute my article in such negative terms. Maybe they are rebutting its political implications, which — if you have chaired the UN Millennium Project as Sachs has — and pinned your legacy on that, must touch a nerve.

Certainly they do not deny the facts underpinning my argument, which when published in PLoS Medicine referenced 41 articles. Sachs and colleagues' reply contains zero references to the literature — zero references to the evidence.

A reply that contains no contrary evidence is not a rebuttal but a polemic.

Science-based development

The Science and Development Network website is not a forum in which to get into a 'he said, she said' exchange of disagreeing views. As its name says, it is the place to consider how scientific tools and reasoning can help achieve development. Three thoughts come to mind.

First, the UN must resist the temptation to treat the MDGs as political playthings for world leaders. They might have been, had the 2000 declaration phrased them as merely qualitative goals. Instead the goals were defined as time-bound, measurable goals.

Second, it is naïve to hail the MDGs as worthwhile goals to spend money on without measuring achievement. Like Sachs and colleagues, I would like to see tens of billions more dollars spent on development each year. Indeed, in 2001, we published a joint plea for increased aid (The Lancet 357, 57).But I do not agree that for such an input of money, there is a guaranteed output of deaths avoided or lives newly dignified.

Measuring the success of initiatives to meet the goals will require redefining or discarding some MDGs that are now immeasurable; vastly greater numbers of demographic surveillance sites in the poorest countries; sample sizes and techniques with adequate sensitivity and specificity, to achieve a statistically significant demonstration of trends over time; and a culture of accountability in the UN backed by external and independent auditing of results.

Contrary to what Sachs and colleagues write, achieving better measurement will not always cost the UN money that it does not have. So although the UN's budget shortfall is real, to argue that the omissions of measurement are the fault of stingy donors is special pleading — a partisan excuse.

For example, the world's best dataset on the extent of malaria was published in Nature this year and had been offered to the World Health Organization (WHO) for free (Nature 434, 214). But for a long time the WHO spurned the data.

Then, just a day after the Nature paper was published, the WHO rushed out its in-house malaria figures in draft form. Not only did the WHO reject an offer of free, reliable, peer-reviewed data, but it wasted its scarce money duplicating that work.

Providing for the poor

Thirdly, we would not be having this debate if it were about rich people. Imagine if the US president set a Millennium Unemployment Goal to halve the number of people without jobs by 2015. Then suppose some years later, an academic asked the government: "So, how much unemployment is there?"

If the government's answer were, "We never measured that, and you're right that we don't know, but shame on you for blaming us", the public outcry would be huge. So would the realisation that the government was unaccountable and disdainful of the people it is meant to protect.

This is exactly where the UN finds itself today over several of its most important MDGs: it pushed for goals that its own scientists knew it could not measure. Largely it gets away with that because world's poorest people are seldom in a position to complain.

Rebuking me for drawing attention to it is shooting the messenger. This does not solve the problem — which Sachs and colleagues concede exists.

We all want the MDGs to succeed, but defending their existence with polemic is not the way. Setting measurable goals, measuring them to guarantee progress, and celebrating the progress as it happens — not just celebrating the goals because they are comforting — is the proper way to dignify and protect the lives of the world's neediest citizens.

Amir Attaran is Canada Research Chair in Law, Population Health, and Global Development Policy, University of Ottawa, Canada.